How did our Earth get here? Or any planet, for that matter? The pessimistic answer: nobody really knows. The optimistic answer: we know more than you think… But the real question is: where did all the planets go?!

For many decades, astronomers have been speculating about how planets form and, lacking observable data, primarily used theoretical models based on fields like geology, thermodynamics and relativity (and more) to predict planetary scenarios. It may come as a surprise to some, but the planets in our solar system are quite poorly understood (in some respects!). Very few probes have been sent to (or near) those planets to make the careful measurements necessary to make strong conclusions about the structure of those planets, their composition, ring structure (if any) and material composition. Even the interior structure of our own Earth has not been fully resolved, let alone how it came to be the way it is.

Which brings us to the critical question of this article: why are some planets missing?! I’d like to start by saying that no one has been stealing planets (although that is an interesting proposition*)... you’ll see what I mean in just a moment!

NASA’s Transiting Exoplanet Survey Satellite (TESS) has been operating for about 1 year looking for exoplanets (planets outside of our solar system). It does this by monitoring the brightness of stars; when the brightness of that star drops periodically, it is deduced that a planet must be transiting that star, periodically blocking the stars’ light to us, because of its regular orbit. Certain properties of those planets can be determined from these transits: the profile of the ‘edge’ of the brightness curve could tell us about whether the planet has an atmosphere or not, the decrease in the star’s periodic brightness can tell us about how close (or how large) the planet is to the star, etc. So far TESS has found 24 planets since April 2018. We would probably expect planets of all sizes to be found (perhaps a Gaussian distribution), right? Wrong! A recent unexpected finding is slowly being confirmed: Planets that are 2 to 4 times the size of Earth are common. However, planets 1.5 to 2 times the size of Earth are rare. Why?

For about a decade, the Kepler Space Telescope scoured the night sky for exoplanets; clocking a whopping 2,662 exoplanets. Within their data, they found that between 1.5 to 2 Earth masses, planets seem to be relatively scarce. This has become known as the Fulton gap; named eponymously after the scientist who first described it in 2017.

A few suggestions have been made to explain why planets between 1.5 to 2 Earth masses are ‘missing’. One suggestion is that there is a mass tipping point (many scientists go for "Goldilocks" terminology...) for maintaining an atmosphere. Only planets above a certain size will maintain an atmosphere, whereas below a certain size it will lose its atmosphere. These mass tipping points could affect other planetary geologic processes that dictate the final mass of a planet. Another theory, introduces tipping points as early as planetary genesis i.e. the final material outcome of a planet is highly sensitive to the initial conditions of that planet’s formation. Another suggestion, is that during the cooling stage of a planets formation it loses mass by its atmosphere evaporating, meaning that different sizes of planets will have a different likelihood of forming. All contenders are possible; but more data is needed to corroborate the observed data with theoretical models to help explain this phenomenon.

Having said this, there are many general rules that can be understood by making rudimentary observations of the planets in our solar system. For example, small rocky planets form closer to our sun, whereas the larger gas giants orbit further away. Tidal forces are partly responsible for whether planetary rocks coalesce to form planets or not (think of the rocky ‘remnants’ of the asteroid belt, or the rocky/icy rings of planets as being unformed moons). Furthermore, the radiative processes imparted by our sun are partly responsible for the formation of gas giants at larger orbital radii, but could this somehow be the cause of the ‘size gap’ of the observed ‘missing planets’?

It’s exciting that TESS is also seeing a gap in the size distribution of planets. However, with only 24 planets detected by it, it will take some time to confirm whether this trend is ‘real’. What is great to see is that the working hypothesis in one experiment (the Kepler Space telescope, and data from 2,662 planets discovered, with a ‘size distribution gap’) is now being tested in a completely different experimental set up (TESS). One of the hallmarks of a good theory is ‘predictive power’. Without being able to test your hypothesis its hard to say anything about… anything, in science. So the results from TESS seem really interesting, in the wake of the prior Kepler results.

The ‘missing planets’ observation is currently an unexplained anomaly. Such results can only mean one thing: a journey in to the unknown. A new way of understanding things, is on the horizon. Exciting times.

*capturing a planet would be awesome. I’m working on a proposal to the Xprize, to secure funding for this project to “end the war against scarcity of resources”... by stealing a planet